netfilter: nf_tables: fix memory leak if expr init fails

If expr init fails then we need to free it.

So when the user add a nft rule as follows:

  # nft add rule filter input tcp dport 22 flow table ssh \
    { ip saddr limit rate 0/second }

memory leak will happen.
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1724,9 +1724,11 @@ struct nft_expr *nft_expr_init(const struct nft_ctx *ctx,
 
 	err = nf_tables_newexpr(ctx, &info, expr);
 	if (err < 0)
-		goto err2;
+		goto err3;
 
 	return expr;
+err3:
+	kfree(expr);
 err2:
 	module_put(info.ops->type->owner);
 err1: